Washington -LRB- CNN -RRB- -- Senate Republicans again Tuesday blocked Democratic legislation that would require greater transparency into who is behind much of the secretive , often negative campaign advertising filling the airwaves this election season .

Supporters of the DISCLOSE Act mustered 53 votes in an effort to advance the bill , but fell short of the 60 needed to break a GOP filibuster . Democrats unanimously supported the bill , while Republicans voted unanimously against it . The legislation , which needs 60 votes to succeed , failed on a 51-44 vote Monday .

The legislation would require certain tax-exempt groups involved in political advertising -- which currently are not required to disclose the names of their donors -- to release the names of those who give donations of $ 10,000 or more . A similar vote Monday ended with the same result .

`` During this election , outside spending by GOP shell groups is expected to top $ 1 billion , '' said Senate Majority Leader Harry Reid , D-Nevada , in a floor speech . `` The names of these front groups contain words like ` freedom ' and ` prosperity . ' But make no mistake : there is nothing free about an election purchased by a handful of billionaires for their own self-interest . ''

Read Reid 's prepared statement

Democrats say the requirement is a much-needed response to the U.S. Supreme Court 's Citizen United decision , which opened the doors to large amounts of campaign spending by corporations , labor unions and other groups .

The political advertising all those donations buy `` threatens to drown out the voice of middle-class families in our democracy , '' argued Sen. Sheldon Whitehouse , D-Rhode Island , a chief sponsor of the bill .

Senate Minority Leader Mitch McConnell , R-Kentucky , led the GOP charge against the bill and accused Democrats of crafting it to favor labor unions -- traditionally Democratic allies -- over other types of donors .

Read McConnell 's statement

`` This legislation is an unprecedented requirement for groups to publicly disclose their donors , stripping a protection recognized and solidified by the courts , '' said McConnell , who has advocated for more disclosure in the past so long as it applies to all big players in campaign financing , such as labor unions and trial lawyers .

McConnell cited a recent Wall Street Journal analysis that found labor unions spent $ 4.4 billion on campaigns between 2005 and 2011 .

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`` Predictably , unions are exempted from the kind of disclosure Democrats now want to impose on others , '' McConnell said .

Democrats disagreed with the Republicans ' assertion that labor unions are exempt from their legislation . However , the concern about labor unions appeared to sway key Republicans such as Sen. John McCain of Arizona , a longtime vocal advocate of campaign finance reform , who said he would vote against the bill for that reason .

Read McCain 's full remarks

`` By conveniently setting high threshold for reporting requirements , the DISCLOSE Act forces some entities to inform the public about the origins of their financial support , while allowing others -- most notably those affiliated with organized labor -- to fly beneath the Federal Election Commission 's regulatory radar , '' McCain said .

McCain argued that local union chapters would not have to disclose payments from individual members if the amount was less than $ 10,000 .

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GOP filibuster blocks DISCLOSE Act again

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The Democratic bill would require certain groups to disclose names of high-paying donors

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Republicans argue it favors labor unions over other types of donors

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The legislation failed in a Monday vote , 51-44